1. Field of the Invention
The present invention relates to a semiconductor device having a circuit constituted of a thin-film transistor (hereinafter referred to as TFT) formed on a substrate having an insulating surface, and to a method of manufacturing the semiconductor device. Specifically, the present invention can be applied to electro-optical devices represented by liquid crystal displays having pixel portions and a driver circuit formed on the periphery of each pixel portion, the pixel portions and the driver circuits being formed on one substrate, and to electrical apparatuses incorporating such electro-optical devices. In this specification, “semiconductor device” denotes a category consisting of all devices capable of functioning semiconductor characteristics, including the above-mentioned electro-optical devices and electrical apparatuses incorporating electro-optical devices.
2. Description of the Related Art
The development of techniques for forming TFTs on a substrate having an insulating surface is being promoted. It is not possible for a TFT using a film of an amorphous semiconductor (typically, amorphous silicon) as an active layer to have a field effect mobility of 10 cm2/Vsec or higher because of electrical and physical factors relating to the amorphous structure, etc. In active matrix liquid crystal displays, therefore, this kind of TFT cannot be used in a pixel portion to form a driver circuit for performing image display, although it can be used as a switching element (pixel TFT) for driving the liquid crystal in a pixel portion. Therefore, to form driver circuits, techniques for mounting driver ICs and the like, based on a tape automated bonding (TAB) or a chip-on-glass (COG) method are presently being used.
On the other hand, a TFT using a semiconductor film including a crystal structure (hereinafter referred to as a crystalline semiconductor) (typically, crystalline silicon film or polycrystalline silicon film) as an active layer can be used in combination with various functional circuits by being formed on one glass substrate together with such circuits, because it can have a high field effect mobility. That is, a shift register circuit, a level shifter circuit, a buffer circuit, a sampling circuit, etc., other than the TFT can be formed in a driver circuit.
An aluminum film having a low resistivity is frequently used as a wiring material with the above-described TFTs.
To realize an active matrix liquid crystal display having a large screen, aluminum (Al), copper (Cu), etc., have been used as wiring materials. This is because there is no suitable low-resistance material comparable to aluminum (Al) and copper (Cu) with respect to use for manufacture of a large-screen display device.
However, these materials are disadvantageously low in corrosion resistance and heat resistance. It is not always preferable to form a TFT gate electrode of one of these materials. Also, each of these materials cannot easily be introduced into a TFT manufacturing process. Conventional TFT manufacturing processes using aluminum as a wiring material have been such that a heat treatment forms hillocks, whiskers, and the like, or causes diffusion of aluminum atoms in a channel forming region, resulting in a TFT operation failure or a deterioration in TFT characteristics.
For this reason, trials have been made to use materials containing tantalum (Ta) or titanium (Ti) as a main constituent in place of aluminum (Al), copper (Cu) and other conventional wiring materials. This is because tantalum and titanium have high heat resistance, although they have a resistivity higher than that of aluminum.
TFTs having an active layer formed of a crystalline semiconductor layer are superior in TFT characteristics. However, a complicated process consisting of a lager number of steps is required for manufacturing of TFTs adapted to various circuits in addition to pixel TFTs. Needless to say, an increase in the number of process steps leads not only to an increase in the cost of manufacturing but also to a reduction in yield.
However, a TFT circuit in a pixel portion and a TFT circuit in a driver circuit do not always operate under the same conditions and, therefore, the characteristics which the TFTs in these circuits respectively need to have differ to some extent. For example, a pixel TFT formed as an n-channel TFT is driven as a switching element to apply a voltage to a liquid crystal. A liquid crystal is driven by an AC voltage and a system for performing frame-inversion drive is used as a liquid crystal drive system in many cases. In this system, a characteristic required of a pixel TFT to limit the power consumption is a sufficiently small off current value (a drain current when the TFT is off). On the other hand, a high drive voltage is applied to a buffer circuit or the like in a driver circuit, and it is necessary to increase the withstand voltage of a TFT in such a circuit to prevent breakdown when a high voltage is applied. Also, to improve the current drive capability, it is necessary to set a sufficiently large on current value (a drain current that flows when the TFT is on).
As a TFT structure devised to reduce the off current, a lightly doped drain (LDD) structure is known. In this structure, a region doped with an impurity element at a low concentration, which is called an LDD region, is provided between a channel forming region and a source region or a drain region formed by doping with an impurity element at a high concentration. A gate-drain overlapped LDD (GOLD) structure in which an LDD structure is provided in a state of overlapping a gate electrode with a gate insulating film interposed therebetween is also known as a means for preventing a reduction in the on current value due to hot carriers. It is known that this structure is effective in limiting hot carrier injection by reducing the high intensity of electric field in the vicinity of the drain and, hence, in avoiding degradation caused by hot carrier injection.
However, pixel TFTs and TFTs in a driver circuit, e.g., a shift register circuit, a buffer circuit, and the like are not always operated in the same biased state. For example, while a high reverse bias voltage (negative voltage in the case of an n-channel TFT) is applied to the gate of a pixel TFT, a TFT in a driver circuit is basically operated without being set in a reverse-biased state. The GOLD structure is advantageously effective in limiting the reduction in the on current value, but it may increase the off current value if the LDD region is only arranged to simply overlap the gate electrode. On the other hand, the ordinary LDD structure is advantageously effective in limiting the off current value but its effect of limiting degradation caused by hot carrier injection is low. These problems have become evident with the improvement in characteristics of crystalline silicon TFTs and with the increase in the required level of performance of active matrix liquid crystal displays. Consequently, to suitably operate TFTs by considering different operating conditions and to prevent the above-described hot carrier effect, it is necessary to optimize the concentration and distribution of an impurity in the LDD region and other factors.